Male and female marsh harriers should be easy to tell apart: the males have grey wing-tips and tails, while the females are mostly brown with distinctive creamy heads. The males also tend to be around 30 percent smaller. But looks can be deceptive. In western France, many of the “female” harriers are actually cross-dressing males that permanently wear the plumage of the opposite sex. Audrey Sternalski has found that this unusual costume allows them to lead more peaceful lives.

Forty percent of male marsh harriers don female costumes, and they start wearing them from their second year of life. Their feathers have the same colours, and they’re smaller in size. Only their irises give them away – they are pale, rather than the ochre-brown of females or the yellow-white of males.

To test the effect of these colours, Sternalski created model harriers and placed them in the territories of real ones. He found that males attacked the male decoys twice as often as either the female or female-like ones. So, by looking like females, male harriers become the beneficiaries of a “non-aggression pact”. They can get access to resources and mates without incurring the wrath of other males. Indeed, Sternalski found that typical males were forced to nest twice as far from another male as the female-like males did.

Sternalski also found that the female-like males almost never attacked male decoys. Instead, they were more likely to attack other females (or female-like males), just as true females are. Not only did they look like females, they behaved like them too.

This raises several questions – are the female-like males simply doing a superficial impersonation, or are they “female” at a deeper physiological level? To find answers, Sternalski now plans to study the genetic basis of the harrier’s female mimicry.

The marsh harrier is one of only two birds whose males permanently don the colours of females. The other – the ostentatious ruff – also uses its disguise to avoid aggressive assaults. They sneak into the territories of more dominant males and surreptitiously mate with the resident females. Such strategies are fairly common in the animal kingdom – they’re found in ants, wasps, fish, and more. In most cases, the deceptive males get some sneaky sex, or avoid attacks from rivals.

But that’s not necessarily the case. In 1985, scientists discovered that some male red-sided garter snakes release a female pheromone that attracts big clusters of up to 17 amorous suitors. By luring these males to him, the female mimic more easily mates with an actual female. The goal seems obvious: distract other males. But the same group later showed that the female-mimics might simply benefit by drawing heat from the writhing balls of other duped males.

This may be a dumb question, but if female mimic snakes are attracting males to it, then how is it getting with the females? Wouldn’t the swarming males around it be an obstacle of sorts? And I’m guessing that the amorous males aren’t inflicting any harm on the female mimic in terms of its aggression either?

This looks like another case of Joan Roughgarden’s idea that a lot of traits we associate with sex are often more strongly associated with social role—that is, the “male” plumage here is really about signalling whether or not the individual is territorial. So in some sense the “female-like” plumage here may not be deceptive, but signalling to territorial males that the individual isn’t interested in driving them off their turf.

In another case (as Roughgarden frames it), both male and female white-throated sparrows come in “aggressive” and “nonaggressive” morphs, with similar plumage regardless of sex—and mated pairs are most successful when one partner is aggressive and the other not, regardless of which sex is the aggressive one.